On Thu, Feb 20, 2025 at 7:40 AM Lokesh Gidra <lokeshgidra@xxxxxxxxxx> wrote: > > On Wed, Feb 19, 2025 at 3:25 AM Barry Song <21cnbao@xxxxxxxxx> wrote: > > > > From: Barry Song <v-songbaohua@xxxxxxxx> > > > > userfaultfd_move() checks whether the PTE entry is present or a > > swap entry. > > > > - If the PTE entry is present, move_present_pte() handles folio > > migration by setting: > > > > src_folio->index = linear_page_index(dst_vma, dst_addr); > > > > - If the PTE entry is a swap entry, move_swap_pte() simply copies > > the PTE to the new dst_addr. > > > > This approach is incorrect because even if the PTE is a swap > > entry, it can still reference a folio that remains in the swap > > cache. > > > > If do_swap_page() is triggered, it may locate the folio in the > > swap cache. However, during add_rmap operations, a kernel panic > > can occur due to: > > page_pgoff(folio, page) != linear_page_index(vma, address) > > > > $./a.out > /dev/null > > [ 13.336953] page: refcount:6 mapcount:1 mapping:00000000f43db19c index:0xffffaf150 pfn:0x4667c > > [ 13.337520] head: order:2 mapcount:1 entire_mapcount:0 nr_pages_mapped:1 pincount:0 > > [ 13.337716] memcg:ffff00000405f000 > > [ 13.337849] anon flags: 0x3fffc0000020459(locked|uptodate|dirty|owner_priv_1|head|swapbacked|node=0|zone=0|lastcpupid=0xffff) > > [ 13.338630] raw: 03fffc0000020459 ffff80008507b538 ffff80008507b538 ffff000006260361 > > [ 13.338831] raw: 0000000ffffaf150 0000000000004000 0000000600000000 ffff00000405f000 > > [ 13.339031] head: 03fffc0000020459 ffff80008507b538 ffff80008507b538 ffff000006260361 > > [ 13.339204] head: 0000000ffffaf150 0000000000004000 0000000600000000 ffff00000405f000 > > [ 13.339375] head: 03fffc0000000202 fffffdffc0199f01 ffffffff00000000 0000000000000001 > > [ 13.339546] head: 0000000000000004 0000000000000000 00000000ffffffff 0000000000000000 > > [ 13.339736] page dumped because: VM_BUG_ON_PAGE(page_pgoff(folio, page) != linear_page_index(vma, address)) > > [ 13.340190] ------------[ cut here ]------------ > > [ 13.340316] kernel BUG at mm/rmap.c:1380! > > [ 13.340683] Internal error: Oops - BUG: 00000000f2000800 [#1] PREEMPT SMP > > [ 13.340969] Modules linked in: > > [ 13.341257] CPU: 1 UID: 0 PID: 107 Comm: a.out Not tainted 6.14.0-rc3-gcf42737e247a-dirty #299 > > [ 13.341470] Hardware name: linux,dummy-virt (DT) > > [ 13.341671] pstate: 60000005 (nZCv daif -PAN -UAO -TCO -DIT -SSBS BTYPE=--) > > [ 13.341815] pc : __page_check_anon_rmap+0xa0/0xb0 > > [ 13.341920] lr : __page_check_anon_rmap+0xa0/0xb0 > > [ 13.342018] sp : ffff80008752bb20 > > [ 13.342093] x29: ffff80008752bb20 x28: fffffdffc0199f00 x27: 0000000000000001 > > [ 13.342404] x26: 0000000000000000 x25: 0000000000000001 x24: 0000000000000001 > > [ 13.342575] x23: 0000ffffaf0d0000 x22: 0000ffffaf0d0000 x21: fffffdffc0199f00 > > [ 13.342731] x20: fffffdffc0199f00 x19: ffff000006210700 x18: 00000000ffffffff > > [ 13.342881] x17: 6c203d2120296567 x16: 6170202c6f696c6f x15: 662866666f67705f > > [ 13.343033] x14: 6567617028454741 x13: 2929737365726464 x12: ffff800083728ab0 > > [ 13.343183] x11: ffff800082996bf8 x10: 0000000000000fd7 x9 : ffff80008011bc40 > > [ 13.343351] x8 : 0000000000017fe8 x7 : 00000000fffff000 x6 : ffff8000829eebf8 > > [ 13.343498] x5 : c0000000fffff000 x4 : 0000000000000000 x3 : 0000000000000000 > > [ 13.343645] x2 : 0000000000000000 x1 : ffff0000062db980 x0 : 000000000000005f > > [ 13.343876] Call trace: > > [ 13.344045] __page_check_anon_rmap+0xa0/0xb0 (P) > > [ 13.344234] folio_add_anon_rmap_ptes+0x22c/0x320 > > [ 13.344333] do_swap_page+0x1060/0x1400 > > [ 13.344417] __handle_mm_fault+0x61c/0xbc8 > > [ 13.344504] handle_mm_fault+0xd8/0x2e8 > > [ 13.344586] do_page_fault+0x20c/0x770 > > [ 13.344673] do_translation_fault+0xb4/0xf0 > > [ 13.344759] do_mem_abort+0x48/0xa0 > > [ 13.344842] el0_da+0x58/0x130 > > [ 13.344914] el0t_64_sync_handler+0xc4/0x138 > > [ 13.345002] el0t_64_sync+0x1ac/0x1b0 > > [ 13.345208] Code: aa1503e0 f000f801 910f6021 97ff5779 (d4210000) > > [ 13.345504] ---[ end trace 0000000000000000 ]--- > > [ 13.345715] note: a.out[107] exited with irqs disabled > > [ 13.345954] note: a.out[107] exited with preempt_count 2 > > > > Fully fixing it would be quite complex, requiring similar handling > > of folios as done in move_present_pte. For now, a quick solution > > is to return -EBUSY. > > I'd like to see others' opinions on whether a full fix is worth > > pursuing. > > > > Thanks a lot for finding this. > > As a user of MOVE ioctl (in Android GC) I strongly urge you to fix > this properly. Because this is not going to be a rare occurrence in > the case of Android. And when -EBUSY is returned, all that userspace > can do is touch the page, which also does not guarantee that a > subsequent retry of the ioctl will succeed. Not trying to push this idea, but I’m curious if it's feasible: If UFFDIO_MOVE fails, could userspace fall back to UFFDIO_COPY? I’m still trying to wrap my head around a few things, particularly what exactly UFFDIO_MOVE is doing with mTHP, as I mentioned in my reply to Suren and you in another email: https://lore.kernel.org/linux-mm/CAGsJ_4yx1=jaQmDG_9rMqHFFkoXqMJw941eYvtby28OqDq+S7g@xxxxxxxxxxxxxx/ > > > For anyone interested in reproducing it, the a.out test program is > > as below, > > > > #define _GNU_SOURCE > > #include <stdio.h> > > #include <stdlib.h> > > #include <string.h> > > #include <sys/mman.h> > > #include <sys/ioctl.h> > > #include <sys/syscall.h> > > #include <linux/userfaultfd.h> > > #include <fcntl.h> > > #include <pthread.h> > > #include <unistd.h> > > #include <poll.h> > > #include <errno.h> > > > > #define PAGE_SIZE 4096 > > #define REGION_SIZE (512 * 1024) > > > > #ifndef UFFDIO_MOVE > > struct uffdio_move { > > __u64 dst; > > __u64 src; > > __u64 len; > > #define UFFDIO_MOVE_MODE_DONTWAKE ((__u64)1<<0) > > #define UFFDIO_MOVE_MODE_ALLOW_SRC_HOLES ((__u64)1<<1) > > __u64 mode; > > __s64 move; > > }; > > #define _UFFDIO_MOVE (0x05) > > #define UFFDIO_MOVE _IOWR(UFFDIO, _UFFDIO_MOVE, struct uffdio_move) > > #endif > > > > void *src, *dst; > > int uffd; > > > > void *madvise_thread(void *arg) { > > if (madvise(src, REGION_SIZE, MADV_PAGEOUT) == -1) { > > perror("madvise MADV_PAGEOUT"); > > } > > return NULL; > > } > > > > void *fault_handler_thread(void *arg) { > > struct uffd_msg msg; > > struct uffdio_move move; > > struct pollfd pollfd = { .fd = uffd, .events = POLLIN }; > > > > pthread_setcancelstate(PTHREAD_CANCEL_ENABLE, NULL); > > pthread_setcanceltype(PTHREAD_CANCEL_DEFERRED, NULL); > > > > while (1) { > > if (poll(&pollfd, 1, -1) == -1) { > > perror("poll"); > > exit(EXIT_FAILURE); > > } > > > > if (read(uffd, &msg, sizeof(msg)) <= 0) { > > perror("read"); > > exit(EXIT_FAILURE); > > } > > > > if (msg.event != UFFD_EVENT_PAGEFAULT) { > > fprintf(stderr, "Unexpected event\n"); > > exit(EXIT_FAILURE); > > } > > > > move.src = (unsigned long)src + (msg.arg.pagefault.address - (unsigned long)dst); > > move.dst = msg.arg.pagefault.address & ~(PAGE_SIZE - 1); > > move.len = PAGE_SIZE; > > move.mode = 0; > > > > if (ioctl(uffd, UFFDIO_MOVE, &move) == -1) { > > perror("UFFDIO_MOVE"); > > exit(EXIT_FAILURE); > > } > > } > > return NULL; > > } > > > > int main() { > > again: > > pthread_t thr, madv_thr; > > struct uffdio_api uffdio_api = { .api = UFFD_API, .features = 0 }; > > struct uffdio_register uffdio_register; > > > > src = mmap(NULL, REGION_SIZE, PROT_READ | PROT_WRITE, MAP_PRIVATE | MAP_ANONYMOUS, -1, 0); > > if (src == MAP_FAILED) { > > perror("mmap src"); > > exit(EXIT_FAILURE); > > } > > memset(src, 1, REGION_SIZE); > > > > dst = mmap(NULL, REGION_SIZE, PROT_READ | PROT_WRITE, MAP_PRIVATE | MAP_ANONYMOUS, -1, 0); > > if (dst == MAP_FAILED) { > > perror("mmap dst"); > > exit(EXIT_FAILURE); > > } > > > > uffd = syscall(SYS_userfaultfd, O_CLOEXEC | O_NONBLOCK); > > if (uffd == -1) { > > perror("userfaultfd"); > > exit(EXIT_FAILURE); > > } > > > > if (ioctl(uffd, UFFDIO_API, &uffdio_api) == -1) { > > perror("UFFDIO_API"); > > exit(EXIT_FAILURE); > > } > > > > uffdio_register.range.start = (unsigned long)dst; > > uffdio_register.range.len = REGION_SIZE; > > uffdio_register.mode = UFFDIO_REGISTER_MODE_MISSING; > > > > if (ioctl(uffd, UFFDIO_REGISTER, &uffdio_register) == -1) { > > perror("UFFDIO_REGISTER"); > > exit(EXIT_FAILURE); > > } > > > > if (pthread_create(&madv_thr, NULL, madvise_thread, NULL) != 0) { > > perror("pthread_create madvise_thread"); > > exit(EXIT_FAILURE); > > } > > > > if (pthread_create(&thr, NULL, fault_handler_thread, NULL) != 0) { > > perror("pthread_create fault_handler_thread"); > > exit(EXIT_FAILURE); > > } > > > > for (size_t i = 0; i < REGION_SIZE; i += PAGE_SIZE) { > > char val = ((char *)dst)[i]; > > printf("Accessing dst at offset %zu, value: %d\n", i, val); > > } > > > > pthread_join(madv_thr, NULL); > > pthread_cancel(thr); > > pthread_join(thr, NULL); > > > > munmap(src, REGION_SIZE); > > munmap(dst, REGION_SIZE); > > close(uffd); > > goto again; > > return 0; > > } > > > > As long as you enable mTHP (which likely increases the residency > > time of swapcache), you can reproduce the issue within a few > > seconds. But I guess the same race condition also exists with > > small folios. > > > > Fixes: adef440691bab ("userfaultfd: UFFDIO_MOVE uABI") > > Cc: Andrea Arcangeli <aarcange@xxxxxxxxxx> > > Cc: Suren Baghdasaryan <surenb@xxxxxxxxxx> > > Cc: Al Viro <viro@xxxxxxxxxxxxxxxxxx> > > Cc: Axel Rasmussen <axelrasmussen@xxxxxxxxxx> > > Cc: Brian Geffon <bgeffon@xxxxxxxxxx> > > Cc: Christian Brauner <brauner@xxxxxxxxxx> > > Cc: David Hildenbrand <david@xxxxxxxxxx> > > Cc: Hugh Dickins <hughd@xxxxxxxxxx> > > Cc: Jann Horn <jannh@xxxxxxxxxx> > > Cc: Kalesh Singh <kaleshsingh@xxxxxxxxxx> > > Cc: Liam R. Howlett <Liam.Howlett@xxxxxxxxxx> > > Cc: Lokesh Gidra <lokeshgidra@xxxxxxxxxx> > > Cc: Matthew Wilcox (Oracle) <willy@xxxxxxxxxxxxx> > > Cc: Michal Hocko <mhocko@xxxxxxxx> > > Cc: Mike Rapoport (IBM) <rppt@xxxxxxxxxx> > > Cc: Nicolas Geoffray <ngeoffray@xxxxxxxxxx> > > Cc: Peter Xu <peterx@xxxxxxxxxx> > > Cc: Ryan Roberts <ryan.roberts@xxxxxxx> > > Cc: Shuah Khan <shuah@xxxxxxxxxx> > > Cc: ZhangPeng <zhangpeng362@xxxxxxxxxx> > > Signed-off-by: Barry Song <v-songbaohua@xxxxxxxx> > > --- > > mm/userfaultfd.c | 11 +++++++++++ > > 1 file changed, 11 insertions(+) > > > > diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c > > index 867898c4e30b..34cf1c8c725d 100644 > > --- a/mm/userfaultfd.c > > +++ b/mm/userfaultfd.c > > @@ -18,6 +18,7 @@ > > #include <asm/tlbflush.h> > > #include <asm/tlb.h> > > #include "internal.h" > > +#include "swap.h" > > > > static __always_inline > > bool validate_dst_vma(struct vm_area_struct *dst_vma, unsigned long dst_end) > > @@ -1079,9 +1080,19 @@ static int move_swap_pte(struct mm_struct *mm, > > pmd_t *dst_pmd, pmd_t dst_pmdval, > > spinlock_t *dst_ptl, spinlock_t *src_ptl) > > { > > + struct folio *folio; > > + swp_entry_t entry; > > + > > if (!pte_swp_exclusive(orig_src_pte)) > > return -EBUSY; > > > > + entry = pte_to_swp_entry(orig_src_pte); > > + folio = filemap_get_folio(swap_address_space(entry), swap_cache_index(entry)); > > + if (!IS_ERR(folio)) { > > + folio_put(folio); > > + return -EBUSY; > > + } > > + > > double_pt_lock(dst_ptl, src_ptl); > > > > if (!is_pte_pages_stable(dst_pte, src_pte, orig_dst_pte, orig_src_pte, > > -- > > 2.39.3 (Apple Git-146) > > > Thanks Barry
